Refractory cement lining for coreless induction furnaces

ABSTRACT

A multi strata refractory lining for coreless induction furnaces, formed from a castable refractory cement. The composition of the cement is preferably a mixture of fused or dead burned spinel, magnesia and/or alumina aggregate, a lesser amount of reactive magnesia, and a small amount of an organic acid. When the cement is cast into the form of a lining in a furnace, cured, dried, and subjected to an initial metal melting run, a ceramically set or sintered crust is formed on the inner surface of the lining and a soft, friable zone adjacent thereto. This combination results in a superior lining in that when the inevitable cracks develop in the furnace lining, they can propagate only through the hard sintered inner facing of the furnace lining, terminating at the soft zone. The failure of the cracks to propagate completely through the lining prevents runout of the molten metal thus greatly extending the life of the lining.

Burrows Aug. 7, 1973 1 REFRMCTORY CEMENT LINING FOR CORELESS INDUCTIONFURNACES [75] Inventor: Owen M. Burrows, Holden, Mass.

Primary Examiner-Roy N. Envall, Jr. Att0rneyRufus M. Franklin [57]ABSTRACT A multi strata refractory lining for coreless inductionfurnaces, formed from a castable refractory cement. The composition ofthe cement is preferably a mixture of fused or dead burned spinel,magnesia and/or alumina aggregate, a lesser amount of reactive magnesia,and a small amount of an organic acid. When the cement is cast into theform of a lining in a furnace, cured, dried, and subjected to an initialmetal melting run, a ceramically set or sintered crust is formed on theinner surface of the lining'and a soft, friable zone adjacent thereto.This combination results in a superior lining in that when theinevitable cracks develop in the furnace lining, they. can propagateonly through the hard sintered inner facing of the furnace lining,terminating at the'soft zone. The failure of the cracks to propagatecompletely through the lining prevents runout of the molten metal thusgreatly extending the life of the lining.

' 3 Claims, 1 Drawing Figure 1 REFRACTORY CEMENT :LINING FOR CORELE SSINDUCTIONv FURNACES BACKGROIjIND THE INVENTION The invention relatestoimetal melting coreless induction furnaces and more particularly torefractory linings therefor.

In recent years the use of induction furnaces for metal melting hasgrown at a rapid rate. As a result, more stringent demands'are beingmade on the refractory linings. At present, linings for corelessinduction furnaces are formed from refractory ramming cements which arecomposed of a refractory aggregate and a bond. Quartzite, calcinedzircon, calcined fire clay, calcined kyanit, calcined or fused alumina,or, calcined or fused magnesia is generally the refractory aggregate.The bond for the aggregate: is clay or a so-called chemical bond such asarises by addition of an inorganic acid to the aggregate. I

Despite the wide use of the foregoing cements, they possess severalrather serious shortcomings as ramming cements for the formation oflinings of coreless induction furnaces. Clay bonded alumina cements aregenerally too bulky for optimum dry ramming, subject to attack by basicslag, and have a propensity to form layers in ramming. Sintered aluminacements, on the other hand, require very high maturing temperatures,shrink and a relatively hard outer zone of cured (hydraulicallysubstantially and permanently upon heating, and are the most serious onewith respect to their use in coreless induction furnace for metalmelting, is that most of these form a lining which is hard and rigidthrough the full thickness of the lining. In the typical operation of acoreless furnace each melt is poured out completely and cold metal isintroduced into the furnace. This facilitates a rapid change in thetemperature in the refractory lining resulting in severe thermalstresses therein. Cracks are initiated in the inner surface of thelining and because the lining is hard and rigid throughout, the crackspropagate outwardly through the full thickness of the lining thusexposing the induction coils to runout of molten metal.

The principal object of the present invention is to provide a refractorycementlining for coreless induction furnaces that is more readily andeconomically installed and which remains free of cracks that traversethe entire thickness of the lining for a substantially greater number ofmelts than prior art cements.

SUMMARY OF THE INVENTION The invention is a cast refractory lining forcoreless induction furnaces. Compositionally, the lining is essentiallyan aggregate of fused and/or dead burned magnesia, alumina, spine], ormixtures thereof, which is bonded with hydraulically set reactivemagnesia containing relatively minor amounts of the magnesium salt of anorganic acid, the organic acid salt being introduced originally as anacid which subsequently reacts with the reactive magnesiato form saidsalt.

Physically, the present lining is made up of a hard sintered inner face,a soft, friable zone adjacent thereto,

nature of the lining is created when the inner face of an already castand cured lining is sintered, preferably by conducting a'metalmeltingrun in the lined coreless induction furnace. Because of the naturaltemperature gradient through such a furnace lining, decreasing greatlyfrom the inner face in contact with the molten metal to the outersurface of the lining, the tri-zone character of the lining appears whenthe lining is subject to an initial metal melting run.

The advantage of the present lining is that when the sintered refractoryface of the lining develops cracks, as it inevitably will from the sharpand suddentemperature drop to which it is subjected when the meltedmetal is poured out, the cracks in the face will stop at the soft,friable zone rather than traversing through the complete thickness ofthe lining which is what occurs with prior art refractory linings. Thusin the present linings, runout of molten metal is prevented by theinherent ability of the lining to prevent cracks from traversingcompletely through. This problem with prior art induction furnacelinings is greatly diminished by surrounding the hard refractory liningwith a layer of loose or unsintered material, thus providing a crackterminating means similar in its function to the soft, friable zone ofthe present invention. The prior art approach, however, is-not amonolithic structure and involves the two separate steps of preparingthe sintered lining and preparing the back-up portion therefor.

As the present lining is subjected to metal melts subsequent to theinitial one, the hard, sintered zone gradually increases in thickness ofthe lining with each run, until finally the entire thickness of thelining is sintered. When this stage in the history of a given lining hasbeen reached, the cracks will traverse completely through the lining andthe furnace should be relined. The lining has the advantage of being acast lining, a unitary or monolithic structure, and capable of longeruseful life than linings for coreless induction furnace heretofore.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevated side view of asection through a coreless induction furnace according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In forming a typical refractorylining of the present invention a castable cement is prepared from afused or dead burned magnesia, alumina, and/or spinel aggregate, lightlycalcined reactive inagnesia, a small quantity of an organic acid, and asufficient amount of water to make the mixture castable. An appropriateamount of the castable cement slurry is poured or scooped into thebottom of the furnace to be lined so as to provide a floor of desiredthickness. This is rodded with, for example, a forked tool or flat barto facilitate removal of entrapped air. An inner form is then locatedconcentrically within the furnace and more cement slurry is poured orcast between the outer wall of the form and the inner wall (which ismade up of the mudded induction coils) thus forming the vertical wall ofthe lining. This too should be rodded to remove air bubbles. In rodding,care should be exercised so as to avoid rodding to such a degree thatgrain separation results.

When the cast wall has stiffened the topping or upper edge of thelining, including the pouring spout, is formed in the conventionalmanner.

The furnace is then covered to prevent excessive evaporation of thewater, and the covered furnace is allowed to stand until the lining (andtopping) become hard. The lining is then dried slowly in a drying ovenat about 200F, or with a lazy gas flame, or heating lamps. The dryingmust be done slowly and at a relatively low temperature to avoid therapid generation of substantial amounts of steam which can seriouslydamage the lining. When dry, the inner form and braces and the likeassociated therewith are removed and the furnace is ready for use i.e. ametal melting run, which is the last step in the preparation of theinvention-lining.

Cold metal is then placedin the furnace and inductive heating iscommenced at a power input low enough so that the rate of heating of themetal does not exceed IF per hour until the temperature reaches about600F. After a 3 to 8 hour soak at 600F the power input is increased sothat the heat-up rate is 200F per hour until the temperature attains1800F where it is held constant for a second soak of 3 to 8 hours. Thenthe temperature is raised further to the desired metal temperature,preferably 100 to 200F above the intended pouring temperature. At thistime more metal is added to bring the level of the molten metal up toabout two inches above the junction of the invention lining and thetopping.

At this point the refractory lining of the present invention is completeand is as schematically shown in FIG. 1. In FIG. 1 the furnace I restingon the refractory brick base 18, the entire assembly being contained ina transite frame or box 24, is constructed of an insulating frame orsupport 16 made of for example wood, to which the induction coils 12 areaffixed The coils are mudded with a plastic cement 14 such as a claybonded alumina. The refractory lining 2 is composed of the topping 20and the lining proper, which is esssentially a monolithic structure butone which is composed of 3 distinct strata or zones. The inner face zone4, which has been or is in contact with the molten metal 10, is hard andsintered. Adjacent to this is the soft friable zone 6. The third zone ofthe lining is the hard but non-' sintered, hydraulically set stratum 8.The tri-zone character of the lining results from the naturaltemperature gradient which is created by melting the metal via corelessinduction heating.

The tri-zo'ne make-up of the lining proper is the esmelting run.However, because of the soft friable zone adjacent to the sintered innerface, the cracks will stop there and will not traverse the entirethickness of the lining as shown at 22 in FIG. 1. Thus dangerous andcostly metal runout to the induction coils is prevented. The prior artalso teaches a method of preventing runout to the coils but this isaccomplished by a two step method viz. the preparation of a crumblybacking zone adjacent to the mudded coils to which is fitted a sinteredrefractory crucible; this is not a monolithic structure.

As the present furnace lining is used in subsequent melting runs, thesintered refractory zone 4 and the soft friable zone 6 gradually movethrough the complete thickness of the lining 2 toward the mudded coils,and the unsintered hydraulically set zone 8 gradually disappears. Whenafter numerous melting runs the entire lining has become sintered itmust then be removed and replaced. This replacement is greatlyfacilitated by 10- eating a sheet of asbestos or the like between therefractory lining 2 and the mudded coils.

In some instances, depending on the composition of the metal to bemelted, it is advantageous to include in the aggregate 0 to 30 percentby weight of magnesia, alumina or zirconia. Thus, where the principalaggregate is fused or dead burned magnesia, then 0 30 percent fused ordead burned alumina or zirconia may be incorporated. Similarly, if theprincipal aggregate is fused or dead burned alumina, then 0 30 percentof fused or deadburned magnesia or zirconia may be advantageouslyincorporated in the aggregate.

The particle size of the aggregate is a size range of graded particlesizes, rather than a particular particle size, for example 6 grit andfiner" is a very commonly used aggregate size. However, the coarsestgrit size in the range is not critical but is usually dictated bycircumstances such as the thickness of the lining. The same is true withrespect to the active magnesia bond. However the latter is desirablysomewhat finer in the coarsest particle size. For most applications, theparticulate materials should have a graded particle size range of 0.5inch and finer.

So far as concerns the relative amounts of aggregate, reactive magnesiabond and organic acid used to make up the castable cement, the preferredcompositional range is:

Parts by Weight aggregate -97 active magnesia 3-20 organic acid 0.5-5

,the lining, as discussed above, the amount of spine-l present willincrease. This is in no way detrimental to the quality of the lining.Where the aggregate is solely fused or dead burned magnesia, then nospinel formation can occur. Similarly, when the aggregate is solelyspinel, then no new spinel formation can occur.

EXAMPLE I A refractory lining for a pound capacity coreless inductionfurnace was prepared as follows:

A thick cement slurry was prepared which consisted of 67 pounds of fusedmagnesia in graded sizes of 6 grit and finer, 25 pounds of fused aluminain graded sizes of 24 grit and finer, 8 pounds of a powdered reactivemagnesia, 0.5 pounds of oxalic acid, and 12.5 pounds of water. Aquantity of the cement slurry was scooped into the bottom of the furnaceso as to provide a floor therein which was approximately 7 inches thick;this was rodded with a forked tool to allow air bubbles to escape. Acylindrical sheet metal form 7.5 inches in diameter and 15 inches highwas placed centrally in the furnace chamber and was braced so as toprevent the form from becoming dislocated when the cement for the wallof the lining is introduced. Cement slurry was then poured, around theform, in increments with each increment being rodded, until the properwall height was attained. This was allowed to stand until the cementslurry stiffened. The topping, including the pouring spout, were formedin place. The entire assembly was then covered to prevent evaporationand allowed to remain covered overnight. The cover was then removed andthe assembly placed in a drying oven where the lining was dried at 200Ffor 24 hours. Subsequent to the drying cycle, the assembly was removedfrom the oven and the cylindrical form and its braces were removed.

The furnace was then filled with cold iron and the power input to thefurnace was regulated and set so as to cause a temperature rise in themetal of about 100F per hour. When the temperature of the metal reached600F it was held constant for 5 hours after which the temperature wasincreased at a rate of 200F per hour until a temperature of l800F wherethe temperature was again held constant for approximately 5 hours. Thetemperature of the metal was further increased at a rate of 200F perhour until the temperature of the metal was lO0- 200F above the intendedpouring temperature and held at that temperature for 2 hours.

At this point, the inventionlining was fully developed as shown in FIG.1, having a hard sintered refractory inner face 4, an adjacent soft,friable zone 6, and a hard but unsintered outer zone 8.

The molten iron was cast into ingots and the furnace was ready forfurther melting runs.

EXAMPLE II A refractory furnace lining wherein the aggregate wasexclusively alumina, was formed in the same manner as described inExample I. The castable cement utilized was made up of 87 pounds offused alumina, pounds of dead burned (calcined) alumina, 7.5 pounds ofreactive magnesia, 0.5 pound of oxalic acid, and l 1 pounds of water.

What is claimed is:

1. In a coreless induction furnace, an essentially monolithic refractorycement lining, the thickness of said lining consisting of:

a. a sintered inner face,

b. a soft, friable zone adjacent to said inner face, and

c. a relatively hard outer zone of cured, but not sintered, refractorycement, said refractory cement consisting essentially of an aggregat eof a dead burned or fused metal oxide selected from the group consistingof magnesia, alumina, and spinel, and mixtures thereof reactive magnesiabond, and 0 to 30 percent by weight of zirconia oxide.

2. The refractory cement lining of claim 1 consisting essentially offrom to 97 percent by weight of fused or dead burned magnesia made up ofgraded particle sizes, the coarsest being approximately 0.5 inch incross section, and 3 to 20% by weight of reactive powdered magnesia. I

3. The refractory cement lining of claim 1 consisting essentially offrom 80 to 97 percent by weight of nonreactive alumina, 3 to 20 percentby weight of reactive magnesia, said alumina and said magnesia beingmade up of graded particle sizes, the coarsest being approximately 0.5inch in cross section.

2. The refractory cement lining of claim 1 consisting essentially offrom 80 to 97 percent by weight of fused or dead burned magnesia made upof graded particle sizes, the coarsest being approximately 0.5 inch incross section, and 3 to 20% by weight of reactive powdered magnesia. 3.The refractory cement lining of claim 1 consisting essentially of from80 to 97 percent by weight of nonreactive alumina, 3 to 20 percent byweight of reactive magnesia, said alumina and said magnesia being madeup of graded particle sizes, the coarsest being approximately 0.5 inchin cross section.